1. Field of the Invention
This invention relates generally to the field of disc drive data storage devices, and more particularly, but not by way of limitation, to a method and apparatus for the detection of certain mechanical defects in a disc drive.
2. Discussion of the Prior Art
Disc drives of the type referred to as "Winchester" disc drives are well-known in the industry. In such devices, one or more rigid discs, coated with a magnetizable medium, are mounted on the hub of a spindle motor for rotation at a constant high speed. Disc drives of the present generation use spindle motors rotating at up to 7200 RPM.
Information is stored on the discs in a plurality of concentric circular tracks by an array of transducers, or heads (usually one per disc surface) mounted for movement to an electronically controlled actuator mechanism. The storing of information on the discs is sometimes also referred to as "writing", and the subsequent retrieval of information from the discs is also called "reading".
Presently, the most commonly used type of actuator mechanism is the rotary voice coil actuator, sometimes referred to as a rotary moving-coil actuator. With this type of actuator, the transducers used to write and read data are mounted via flexures at the ends a plurality of head arms which project radially outward from a substantially cylindrical actuator body. The actuator body is journaled via ball bearing assemblies to rotate about a pivot shaft which is mounted to the disc drive housing at a position closely adjacent the outer extreme of the discs. The pivot shaft is intended to be in parallel with the axis of rotation of the spindle motor and the discs. The transducers will thus move in a plane parallel with the surfaces of the discs.
A coil is mounted on the side of the actuator body opposite the head arms. The coil is mounted so as to be immersed in the magnetic field of an array of permanent magnets which are in turn mounted to the disc drive housing. When controlled DC current is passed through the coil, an electromagnetic field is set up which interacts with the magnetic field of the permanent magnets and causes the coil to move relative to the permanent magnets in accordance with the well-known Lorentz relationship. As the coil moves relative to the permanent magnets, the actuator body pivots about the pivot shaft and the heads are moved across the disc surfaces.
Control of the movement of the heads is achieved with a closed loop servo system and details of such a servo system can be found in U.S. Pat. No. 5,262,907 issued to Duffy et al., assigned to the assignee of the present invention and incorporated herein by reference. In such a system, position or servo information is prerecorded on at least one surface of one of the discs. The servo system can be either a "dedicated" servo system, in which one entire disc surface is prerecorded with the servo information and a dedicated servo head is used to constantly read the servo information, or an "embedded" servo system, in which servo information is interleaved with user data and intermittently read by the same heads used to read and write the user data.
With either a dedicated or embedded servo system, it is common that the servo circuitry produce a servo position error (SPE) signal which is indicative of the position of the head relative to the center of a track. The identity of the particular track, as well as other information relating to the circumferential position of the head on the track, is included, along with other information, in the prerecorded servo information. Thus, when the heads are following a desired track, the SPE is essentially at a zero value. The SPE is fed back to circuitry used to control current through the coil of the actuator. Any tendency of the heads to deviate from true track center causes the SPE to change from its zero value. The SPE is a bipolar analog signal, meaning that deviation of the head position away from track center in a first direction will produce a SPE of a first polarity, while movement of the heads off track center in the opposite direction will produce an SPE of the opposite polarity, and the greater the distance of the head from track center, the greater the magnitude of the SPE signal. It should be noted that the SPE signal relates to each track centerline, and, as such, when the actuator is seeking from one track to another, the SPE signal switches from maximum offset value from a first track in a first direction to maximum offset value from a second track in the opposite direction as the moving head passes the midpoint between the first and second tracks.
In the manufacture of disc drives, it is not unusual for tens of thousands of disc drive units to be fabricated daily. With such high numbers of disc drives being made, it is apparent that a certain number of units will fail to meet the design specifications, due to faulty components, improper assembly, contamination, and other elements familiar to those of skill in the art. While every effort is made by disc drive manufacturers to minimize these defective units and assembly errors, a small percentage of defective units will occur. When the defect is introduced into the unit at an early stage in the manufacturing process, the fault may not be detected until a much later stage of the process. Such a delay in the detection of defective assemblies can result in a significant amount of labor costs when taken over the large numbers of units being manufactured.
It has been found that several mechanical defects that can commonly be introduced into the assembly of a disc drive can be closely correlated to the introduction of susceptibility of the unit to resonances at fixed "marker" frequencies. This correlation has come about empirically with the experience of building hundreds of thousands of identical products. With this knowledge, it follows that if the disc drive units can be tested for resonance at the marker frequencies, early detection of the manufacturing defects is possible.
It has been found that resonant frequencies in a mechanical structure can sometimes be identified through the use of a frequency analyzer which, once properly connected to the structure to be tested, injects energy at a selected frequency and then evaluates the structure for gain in the energy which would be indicative of resonance. While the use of a frequency analyzer as an engineering diagnostic tool is well known in the industry, it does have several drawbacks which make such use impractical for large-scale implementation in disc drive manufacturing test operations. Firstly, a frequency analyzer is a complex and expensive piece of diagnostic test equipment, costing several thousand dollars per unit. In a manufacturing environment producing tens of thousands of units per day, a large number of frequency analyzers would be needed in order to provide adequate test capability for the quantity of drives being manufactured, resulting in economically prohibitive capital costs for the manufacturer. Secondly, connecting an analyzer to each structure to be tested and performing the test would require both an operator and a significant amount of time, two elements antithetical to such a high volume production environment. Thirdly, the implementation of automated test result reporting and evaluation with such discrete test equipment would be difficult and resource intensive.
It has also been found that testing for sympathetic resonances in a structure can be accomplished by mounting the unit to be tested to a vibration table, and then injecting either sinusoidal or random vibration energy into the unit during operation and then monitoring for resonant frequencies using suitable test equipment. Again, such a method, although useful during development of a disc drive, would be economically impractical for implementation during large scale manufacture due to capital equipment and resource requirements.
It would, therefore, be desirable to provide a method and apparatus for testing for mechanical defects in disc drives by detecting resonances at corresponding marker frequencies, and culling out units failing the test procedure for repair or remanufacture, while allowing passing units to continue onward in the manufacturing process. It would also be preferable if the test methodology involved a minimum of cost, both in human operator time and capital investment.